Yes and perhaps it could decay further (not just by the -3 subtraction I mentioned but even automatically in some cases (e.g. -1 every week after 4 weeks etc.)).
Regarding the current vault: it doesn't have to exist at all; it could just be replaced by this; it's 1 item per week from each one of them anyway.
You could have both system of course though: I guess the devs might want to reduce the total loot dropped across the game somehow.
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It's possible, but without argument against it, I don't see the issue.